Hydrocarbon compounds are useful for a number of purposes. In particular, hydrocarbon compounds are useful, inter alia, as fuels, solvents, degreasers, cleaning agents, and polymer precursors. The most important source of hydrocarbon compounds is petroleum crude oil. Refining of crude oil into separate hydrocarbon compound fractions is a well-known processing technique that can be accomplished by a variety of different methods.
Crude oils range widely in their composition and physical and chemical properties. Crude oil with a similar mix of physical and chemical characteristics, usually produced from a given reservoir, field or sometimes even a region, constitutes a crude oil “stream.” Most simply, crude oils are classified by their density and sulfur content. Less dense (or “lighter”) crudes generally have a higher share of light hydrocarbons—higher value products—that can be recovered with simple distillation. The denser (“heavier”) crude oils produce a greater share of lower-valued products with simple distillation and require additional processing to produce the desired range of products. Heavy crudes are also characterized by a relatively high viscosity and low API gravity (generally lower than 25°) and high percentage of high boiling components (>975° F.).
Additionally, some crude oils also have a higher sulfur content, an undesirable characteristic with respect to both processing and product quality. The quality of the crude oil dictates the level of processing and re-processing necessary to achieve the optimal mix of product output.
In the last two decades, the need to process heavier crude oils has increased. Refined petroleum products generally have higher average hydrogen to carbon ratios on a molecular basis. Therefore, the upgrading of a petroleum refinery hydrocarbon fraction is classified into one of two categories: hydrogen addition and carbon rejection. Hydrogen addition is performed by processes such as hydrotreating and hydrocracking. Carbon rejection processes typically produce a stream of rejected high carbon material which may be a liquid or a solid; e.g., coke.
To facilitate processing, heavy crudes or their fractions are generally subjected to thermal cracking or hydrocracking to convert the higher boiling fractions to lower boiling fractions, followed by hydrotreating to remove heteroatoms such as sulfur, nitrogen, oxygen and metallic impurities.
Further information on hydrotreating catalysts, techniques and operating conditions for residue feeds may be obtained by reference to U.S. Pat. Nos. 5,198,100; 4,810,361; 4,810,363; 4,588,709; 4,776,945 and 5,225,383 which are incorporated herein for this teaching.
Crude petroleums oils with greater amounts of impurities including asphaltenes, metals, organic sulfur and organic nitrogen require more severe processing to remove them. Generally speaking, the more severe the conditions required to treat a given feedstock (e.g. higher temperature and pressures), the greater the cost to build and operate the overall plant.
Worldwide, fixed-bed reactors are utilized considerably more than ebullated-bed reactors. The fixed-bed system is used for lighter, higher quality feedstocks and is a well understood system. Fixed-bed systems are used mostly for naphtha, mid-distillate, atmospheric and vacuum gas-oils, and atmospheric residua treatment.
However, as the feedstock becomes heavier, has a greater level of impurities, or requires more severe conversion levels, the fixed-bed system becomes less effective and less efficient. In these cases, the ebullated-bed reactor systems are better suited for residue processing.
In general, ebullated-bed reactors are utilized to process heavy crude oil feed streams, particularly those feeds with high metals content and high Conradson carbon residue (“CCR”). The ebullated-bed process comprises the passing of concurrently flowing streams of liquids, or slurries of liquids and solids, and gas through a vertically elongated fluidized catalyst bed. The catalyst is fluidized and completely mixed by the upwardly flowing liquid streams. The ebullated-bed process has commercial application in the conversion and upgrading of heavy liquid hydrocarbons and converting coal to synthetic oils.
The ebullated-bed reactor and related process well-known to those skilled in the art and is generally described in U.S. Pat. No. 25,770 to Johanson, which is incorporated herein by reference. Briefly, a mixture of hydrocarbon liquid and hydrogen is passed upwardly through a bed of catalyst particles at a rate such that the particles are forced into random motion as the liquid and gas pass upwardly through the bed. The catalyst bed motion is controlled by a recycle liquid flow so that at steady state, the bulk of the catalyst does not rise above a definable level in the reactor. Vapors, along with the liquid which is being hydrogenated, pass through the upper level of catalyst particles into a substantially catalyst free zone and are removed from the upper portion of the reactor.
Ebullated-bed reactors are generally operated at relatively high temperatures and pressures in order to process these heavy feedstocks. Since such operating parameters substantially increase the cost of designing and constructing the reactors, it would therefore be advantageous to have a system wherein the overall design and manufacturing costs were optimized for specific feedstocks or feedstock components. This optimization would result in a lower initial investment and lower annual operating costs.
Typically, multi-stage ebullated-bed overhead streams processing atmospheric or vacuum residues are combined and sent to additional separation steps including the recovery of light liquids and preparation of a recycle gas which contains any unreacted hydrogen. However, this is not thermally efficient since it requires the streams to be depressurized, cooled down and fractionated, resulting in energy loss.
Alternatively, the combined separator overheads containing significant unreacted hydrogen could be sent to a fixed-bed or ebullated-bed hydrotreater or hydrocracker to hydroprocess the liquids contained in the high pressure vapor plus any external or recycle distillates or VGO. However, even a small amount of entrained vacuum residue and/or fines would render a fixed-bed incapable of processing this feed. Moreover, if the feedrate is high, and if there are high amounts of external streams also requiring hydroprocessing, a single ebullated-bed reactor may not have sufficient capacity to hydroprocess the streams.
It would be therefore desirable to have a configuration which effectively integrates the petroleum atmospheric or vacuum residue hydrocracking and the vacuum gas oil hydrotreating/hydrocracking. Moreover, it would be highly desirable to have a configuration that overcomes the flowrate limitations of conventional designs described above. The present invention overcomes such limitations.
The term “vacuum gas oil” (VGO) as used herein is to be taken as a reference to hydrocarbons or hydrocarbon mixtures which are isolated as distillate streams obtained during the conventional vacuum distillation of a refinery stream, a petroleum stream or a crude oil stream.
The term “naphtha” as used herein is a reference to hydrocarbons or hydrocarbon mixtures having a boiling point or boiling point range substantially corresponding to that of the naphtha (sometimes referred to as the gasoline) fractions obtained during the conventional atmospheric distillation of crude oil feed. In such a distillation, the following fractions are isolated from the crude oil feed: one or more naphtha fractions boiling in the range of from 90 to 430° F. one or more kerosene fractions boiling in the range of from 390 to 570° F. and one or more diesel fractions boiling in the range of from 350 to 700° F. The boiling point ranges of the various product fractions isolated in any particular refinery will vary with such factors as the characteristics of the crude oil source, refinery local markets, product prices, etc. Reference is made to ASTM standards D-975 and D-3699-83 for further details on kerosene and diesel fuel properties.
The term “hydrotreating” as used herein refers to a catalytic process wherein a suitable hydrocarbon-based feed stream is contacted with a hydrogen-containing treat gas in the presence of suitable catalysts for removing heteroatoms, such as sulfur and nitrogen and for some hydrogenation of aromatics.
The term “desulfurization” as used herein refers to a catalytic process wherein a suitable hydrocarbon-based feed stream is contacted with a hydrogen-containing treat gas in the presence of suitable catalysts for removing heteroatoms such as sulfur atoms from the feed stream.
The term “hydrocracking” as used herein refers to a catalytic process wherein a suitable hydrocarbon-based feed stream is contacted with a hydrogen-containing treat gas in the presence of suitable catalysts for reducing the boiling point and the average molecular weight of the feed stream.